All the time UE makes a measurements
This is to trigger Handover to different TA or to Inter-RAT Handover or going few levels down to to monitor if they dont have to change the transmitting power.
I'm not the radio person, but I'm sure one of the logical channels provides the information on what neighboring eNBs are serving.
Information like PLMN-ID, TAC/TAI and probably many many more.
During the never-ending measurements UE is aware of neighboring cells, their GCI and TAI/TAC.